1. Field of the Invention
This invention relates to an optical tomography system for obtaining an optical tomographic image by measurement of OCT (optical coherence tomography) and a method of adjusting quality of an image obtained by a tomography system.
2. Description of the Related Art
When obtaining tomographic images of living tissue, there has sometimes been used optical tomography systems employing OCT measurement. In such an optical tomography system, low coherence light emitted from a light source is divided into measuring light and reference light and the measuring light is projected onto the object of measurement, while the reflected light from the object of measurement when the measuring light is projected onto the object is combined with the reference light, and a tomographic image is obtained on the basis of the intensity of the interference light of the reflected light and the reference light. See, for instance, Japanese Unexamined Patent Publication Nos. 2000-097845, 2000-126188 and 2000-262461.
In the optical tomography system, there are also known systems using the TD-OCT (time domain OCT) measurement, where the measuring position in the depth direction of the object is changed by changing the optical path length of the reference light.
Further, recently, as a system of rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed an SD-OCT system using SD-OCT (spectral domain OCT) measurement. In the SD-OCT measurement, a tomographic image is formed without scanning in the direction of depth, by dividing broadband, low coherence light from a light source into measuring light and reference light by the use of a Michelson interferometer, causing the reflected light returning from the object when the measuring light is projected onto the object to interfere with the reference light and carrying out Fourier analysis on each channeled spectrum obtained by decomposing the interference light of the reflected light and the reference light into frequency components.
Whereas, as a further system for rapidly obtaining a tomographic image without changing the optical path length of the reference light, there has been proposed a system using SS-OCT (swept source OCT) measurement. In the SS-OCT system, a tomographic image is obtained on the basis of an intensity of reflected light in a position in the direction of depth of the object by sweeping the frequency of the laser beam emitted from the light source to cause the reflected light and the reference light to interfere with each other at various wavelengths and carrying out a Fourier analysis on the interference spectrum for a series of wavelengths to obtain the intensity of the reflected light in the position in the direction of depth of the object.
When each of the optical tomography systems described above is applied to an endoscope, a laser is generally used as the light source and an optical fiber is generally employed to guide light to a body cavity. Accordingly, problems related to polarization arises as follows.
Because they have polarization characteristics, the optical parts used in the above systems exhibit different transmissivities, reflectances, branching ratios and the like depending on the direction in which the incident light is polarized. Especially, when the light incident to the optical parts substantially comprises only linearly polarized light polarized in one direction, the influence of the polarization characteristics appears large. When the direction of polarization of the incident light changes, the influence of the polarization is enlarged.
An optical fiber is necessarily folded or twisted when it is inserted into a body cavity. Further, temperature changes of the fiber inherent to insertion into a body cavity occur. In a single-mode fiber which is generally used for an endoscope, the direction of polarization of light propagating through the fiber cannot be preserved. Accordingly, the state of polarization of light propagating through the fiber changes due to stress by folding or twisting, changing factors such as a temperature change or vibration. That is, light propagating through the fiber constantly fluctuates in its state of polarization.
When light which fluctuates in its state of polarization impinges upon the optical parts, the signal level to be detected by a detector by way of the optical parts fluctuates, S/N ratio deteriorates, and values different from the inherent measured values can be obtained. As a result, the quality of the tomographic image can deteriorate, for instance, the tomographic image can be rough and those which is to be distinguished cannot be distinguished from each other. Thus, the change in the image quality due to fluctuation in the state of polarization is especially enlarged when the light is linearly polarized light.
When the reflected light from the object and the reference light are both perfectly linearly polarized, and the directions of polarization are perpendicular to each other, there arises an event where no interference can take place and no tomographic image can be obtained. On the other hand, when each of the reflected light and the reference light is elliptically or circularly polarized, since the interference components of each light perpendicular to each other cannot be nullified even if the state of polarization of each polarized light somewhat changes, and accordingly, an event where no interference can take place seldom occurs and the signal level only somewhat changes, whereby the influence is relatively small.
In Japanese Unexamined Patent Publication No. 2000-262461, there has been disclosed to use on the front end of the probe an element such as a Faraday rotator which rotates the plane of polarization by 45° in order to compensate for change of the interference light due to change of the refractive index caused by curving of the optical fiber and to use a polarization plane controller in order to maximize the interference intensity of the reflected light from the object with the reference light by equalizing directions of polarization of the reflected light and the reference light. However, in order to dispose a Faraday rotator on the front end of the thin probe which is to be inserted into a body cavity, it is necessary to miniaturize the Faraday rotor. The Faraday rotors which can be miniaturized are limited in their wavelength and are inadequate to the above optical tomography system. The polarization plane controller has drawbacks that it acts at low speed since it is mechanically driven, adds to the size of the system, are instable since it is high in the sensitivity and requires adjustment by a man each time the state of propagation in the fiber changes, and is not practical.